At the present time, electronic timepieces which employ a stepping motor to drive time indicating means such as time indicating hands and a calendar display device, are in widespread use. In such a timepiece, the question of reducing battery power consumption as far as possible is extremely important, due to the fact that reduced consumption enables battery size, and hence timepiece size overall, to be reduced. This is of course advantageous in the case of an electronic wristwatch. However, in the case of a timepiece having an auxiliary indicating mechanism, such as a date display, it becomes difficult to arrange that the battery consumption will be held to a suitable minimum value, due to the fact that each time the date display mechanism is driven by the timepiece gear train, the load applied to the stepping motor increases significantly above its normal level. Thus, if the level of drive power applied to the stepping motor during each drive pulse is set to be sufficient for normal operation, this may be insufficient to consistently drive the timepiece under a heavy load condition. Similarly, if the level of drive power is set to be sufficient for operation under the heavy load state, this will result in excessive and wasteful power consumption during the normal, light load condition. In view of this, various schemes have been proposed for detecting the load currently being applied to the stepping motor, and for controlling the power of the drive pulses applied to the motor in accordance with the results of this detection, so that the drive power is reduced when a heavy load is applied and is increased when a light load is applied to the motor. However, accurate realization of such schemes in the case of mass-production timepieces has proved to be extremely difficult. While it may be possible to arrange for such a system to operate in a satisfactory way for a particular stepping motor and circuit, variations in the components of the stepping motors, and variations of the threshold voltage of circuit components such as inverters used for load detection purposes make it difficult or impossible to obtain satisfactory and uniform results on a mass production basis. In general, the case of detecting an increase in load and providing an increase in the drive power to meet this increase can be quite easily accomplished. However, the case of changing from high-power drive to low power drive, when the load on the stepping motor subsequently is reduced, causes serious problems, with a conventional drive power control system. Because of such difficulties, it has been proposed to provide timer means for controlling the duration of an increased drive power status. With this, as soon as a high load on the stepping motor is detected, the drive power is increased for a predetermined period, set by the timer means. When this time has elapsed, then a return to the low-power drive status is performed. Such a system has the disadvantage that power consumption is not minimized to the greatest possible degree, because of the fact that no attempt is made to detect a change from the high load to the low load condition. For example, when an increase from the light load to the heavy load condition is detected, then it is desirable that the drive power be immediately increased. In other words, if a low power drive pulse has been applied to the stepping motor, and if immediately thereafter it is detected that the motor is in the heavily loaded state, then it is usually necessary to apply a high power drive pulse to the motor immediately after that low-power drive pulse, in order to ensure that the motor will actually be stepped, with the heavy load applied to it. With a system employing a timer, then if the timer is set for too short a period of high-power drive, and if the motor continues to operate under the heavy load condition, the result will be that pairs of drive pulses, each consisting of a low power drive pulse followed by a high power drive pulse will be produced, after the time set by the timer has expired and while the heavy load is still being applied to the motor. This is obviously wasteful of power. On the other hand, if the time is set to be too long, then the motor will continue to be driven at the high power level for the entire duration of that time, even in the case of the high load condition being applied only momentarily. This again is a waste of battery power.
With a stepping motor drive control system according to the present invention, these disadvantages of prior art systems are effectively eliminated. This is done by arranging to selectively drive the stepping motor at one of at least three different power levels, in accordance with the load currently being applied to the motor. Immediately a change in the load is detected, a change is made to either the next lower or the next higher drive power level. Load level detection is accomplished by momentarily placing the drive coil of the stepping motor in an open circuit condition for a brief time interval immediately following a drive pulse, and detecting the level of a voltage induced in the coil at that time. It is an important feature of the present invention that the timing of these detection time intervals is varied in accordance with the current drive power status of the stepping motor, as is explained in detail with respect to the preferred embodiment. Use of a drive control system according to the present invention ensures that, each time a change in load level on the motor is detected, no more than a single excessive drive pulse will be produced (for example, a high power drive pulse immediately succeeding a low power drive pulse). Wasteful consumption of battery power is thereby considerably reduced by comparison with prior art systems for stepping motor drive power control in an electronic timepiece. In addition, a drive control system according to the present invention can operate successfuly in spite of wide variations in the characteristics of components in the stepping motor and in the detection circuitry.